An important aspect of digital circuit testing is proper probing of a target system. As an example, logic analysis seeks to make measurements of the target system at speed without affecting target system circuit operation. Ideally, probe apparatus samples circuit voltages exactly as they would as they appear during unprobed operation of the target system. As frequency of operation of the target systems increases, however, it becomes more difficult to probe the target system without adding parasitic impedances to the target system circuits that affect its operation.
A known style of probe apparatus that is suitable for circuit test is a “snoop probe”. With specific reference to FIG. 1 of the drawings, an example of the snoop probe is shown whereby one or more communication lines 101 of the target system circuit is interrupted and the snoop probe in interposed between two ends of the interrupted line. The snoop probe provides a through connection for the interrupted communication line 101 to permit operation, but also has a parallel sampling circuit for probing signal activity on the line. The snoop probe includes a through circuit portion with a tip resistor 102 connected in parallel to the through circuit. The tip resistor 102 provides isolation between the through circuit portion and a measurement device 103. A probe cable 104 connected to the tip resistor 102 brings a sampled signal to the test equipment 103. The test equipment 103 is terminated in a termination impedance. Typically, an impedance of the tip resistor is 5-10 times that of the termination impedance. Advantageously, the snoop probe provides passive probing and isolation from the test equipment. A disadvantage of the known snoop probe is that the test signal as seen by the measurement device is significantly attenuated by a voltage division ratio of the termination impedance relative to the impedance of the tip resistor 102 added to the termination impedance. Because of the voltage division ratio, the test equipment also sees an attenuated slew rate of the signals being measured. Therefore, the effective conversion of noise voltage is similarly slowed, which contributes to jitter in the measurement. The larger the isolation resistor, the better the isolation of the target system from the measurement device 103, but the more the slew rate is attenuated and the larger the apparent measurement jitter. As one of ordinary skill in the art appreciates, the converse is also true. The jitter measurement may be improved with a smaller tip resistor impedance, but at the expense of isolation of the target system circuit 108 from the measurement device 103. Another disadvantage of the snoop probe in that parasitic capacitance that is inherent in the tip resistor 102 causes the isolation to decrease as test signal frequencies increase.
There is a need for an improved probe to provide sampling of signals in a target system without affecting its operation.